The invention relates to an electronic flash device for the purposes of photographic exposure. It includes a flash tube and a storage capacitor dischargeable through the flash tube, a light measuring unit to detect the amount of light reflected by the object of the exposure to produce a signal upon measuring a predetermined amount of light, a flash limiting unit to discontinue the emission of light by the flash tube upon receiving the signal of the light measuring unit and a device to block external flashes. The blocking device blocks the light measuring unit and permits light measurement only during the emission of light by the flash tube. The light measuring unit comprises a photodetector which may comprise a phototransistor. An integrating capacitor is connected in parallel to the base-emitter region of the phototransistor and the signal for the flash limiting unit is taken off a resistor connected to the emitter of the phototransistor.
In a known electronic flash device of this type, such as described in German Offlegungsschrift DT-OS 24 35 235 incorporated herein by reference, the base of the phototransistor is connected with zero potential by means of an NPN transistor. The base of the transistor is connected permanently to a positive potential through a resistor so that the transistor is constantly conducting and the integrating capacitor is discharged. At the onset of the emission of light by the flash tube, a negative pulse is applied to the base of the transistor through an additional resistor, which causes the base to block the transistor, i.e. renders it non-conducting. As a result, the integrating capacitor is charged until the base-emitter region of the phototransistor becomes conductive. Because of the current amplification by the phototransistor, the emitter current rises sharply and a signal for the flash limiting unit may be taken from the emitter.
Thus, while the transistor is conducting, any current generated by the incidence of light upon the phototransistor in the collector-base region of said phototransistor is conducted off through the collector-emitter region of the conducting transistor, thereby preventing an increase in the potential on the integrating capacitor which would otherwise possibly lead to a full switching of the phototransistor and the production of a signal at the emitter. The transistor thus constitutes the external flash blocking means, which allows the charging of the integrating capacitor only during the emission of light by the flash tube by applying a negative pulse to the base of the transistor during the emission of light by the flash tube, said negative pulse causing the blocking of the transistor.
An external flash as utilized herein means simply any light received by the measuring unit which is not generated originally by the electronic flash device. To insure the accurate operation of this external flash blocking means and to prevent it from causing a false effect on the results of measurements by the light measuring unit, the control current which maintains the transistor in a permanently conducting state, i.e. the base current of the transistor, must be as intense as possible, because the collector current of the transistor increases with the intensity of the base current. If during the blocking phase of the external flash blocking means, a strong flash of light is incident upon the phototransistor, then, in order to prevent the charging of the integrating capacitor, the entire base-collector current of the phototransistor must be conducted over the transistor. This, however, is possible only if the transistor is controlled at a saturation level by means of a high control current. A partial charge on the capacitor even if very slight, due to a low level of the control of the transistor, would, in the case of a flash exposure released immediately following the external flash, cause a false result of the measurement by the light measuring unit. The light measuring unit would thus be induced to release a premature signal to the flash limiting unit, causing the latter to terminate the emission of light by the flash tube prematurely. The result would be underexposure of the photographic image.
Because the transistor must be nearly always in a conducting state and is blocked for a short period of time only during the emission of light by the flash tube, such a high intensity control current for the transistor represents an additional load on the battery or charging device. Because the control current for the transistor is usually supplied by the storage capacitor of the flash unit, this procedure results in an additional discharge of the storage capacitor of the flash unit. This is particularly the case where so-called RE circuits are used in flash devices. In such devices, in order to obtain short flash sequences by means of a switching thyristor arranged in series with the flash tube which is blocked by the flash limiting unit to terminate the emission of the flash, only sufficient energy is taken from the storage capacitor to obtain the exact exposure of the photographic image. This additional constant consumption of current by the transistor leads to a reduction of the charge storage available for the flash tube, in turn resulting in longer flash sequence times since, with an identical light volume being emitted by the flash tube, the storage capacitor must be recharged more frequently.